Two million people are at risk from a toxic fog that has descended over Utah as pollution levels in Salt Lake City reach an all-time high.

The icy fog has smothered the Salt Lake Valley for several weeks and trapped lung-busting soot emitted in the city underneath.

The level of pollution in the city is now the highest in America according to the Environmental Protection Agency and medical experts are now urging residents to stay indoors.

The smog over Salt Lake City, which is surrounded by the Wasatch and Oquirrh mountain ranges, has been caused by a temperature inversion where warmer air has created a barrier above cold fog so everything underneath is trapped.

Off the charts: There are now 130 micrograms of soot per cubic meter in Salt Lake City – three times the federal clean-air limit

Snow cover on the surrounding mountains has amplified the phenomena, leaving Salt Lake City at just 18 degrees while Park City basks in sunny 43-degree weather.

The warmer air aloft has acted like a lid on the frigid valley air, leaving it with no place to go.

The pollution is now getting so bad that more than 100 Utah doctors are calling for authorities to immediately lower highway speed limits, curb industrial activity and make mass transit free for the rest of winter.

Doctors say the microscopic soot – a shower of combustion particles from tailpipe and other emissions – can tax the lungs of even healthy people.

‘We’re in a public-health emergency for much of the winter,’ said Brian Moench, a 62-year-old anesthesiologist and president of Utah Physicians for a Healthy Environment, which delivered the petition demanding action at the Utah Capitol.

The greater Salt Lake region had up to 130 micrograms of soot per cubic meter – more than three times the federal clean-air limit, according to the U.S. Environmental Protection Agency.

And for two million Utah residents, there is no escape from the fog, except to the snow-capped mountains surrounding the city or resort towns like Park City, where the Sundance Film Festival is under way.

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